Doctor Who star David often talks about his time on the BBC One sci-fi TV show and holds his time as the Doctor as a special time in his career.
Reflecting on his time in charge of the TARDIS, the actor has revealed how one of his companions was almost replaced.
Fans will remember his companion Donna, who was played by Catherine, turned up in the 2006 Christmas special The Runaway Bride.
However, she ended up reprising her role in the 2008 series despite showrunner Russell T. Davies having another actress in mind.
In his book, The Writer's Tale, Russell explained Donna wasn't going to return and David's new companion was going to be called Penny.
And, David and Catherine spoke about this on his podcast David Tennant Does A Podcast With….
He went into detail about how they were looking for another actress because they didn’t expect Catherine to say yes to returning to the show.
"You did this one episode, and then you came back as a regular, which none of us were expecting [you] to say yes to," the actor remarked.
